The disclosure of Japanese Patent Application No. 2001-163929 filed on May 31, 2001, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a drive control apparatus and method for controlling driving of an alternating current motor (hereinafter simply referred to as xe2x80x9cAC motorxe2x80x9d).
2. Description of Related Art
For driving an AC motor by using a DC power supply, it is widely known to apply a voltage signal of pulse-width modulated (PWM) waveform to the AC motor by using an inverter. However, the PWM waveform voltage is utilized by the AC motor with a relatively low efficiency. Thus, the AC motor to which the PWM waveform voltage is applied cannot produce sufficiently high output or power in a high-speed rotation region.
In view of the above problem, another technology is known in which a voltage signal of rectangular waveform is applied to the AC motor so as to drive/rotate the AC motor. This technology makes it possible to increase power in a high-speed rotation region, and eliminates the need to supply a large amount of field-weakening current to the motor while it is rotating at a high speed, resulting in a reduction in copper loss. Furthermore, the technology makes it possible to reduce the number of times of switching in the inverter, and to thus reduce or suppress switching loss.
FIG. 5 is a block diagram showing the arrangement of a known drive control apparatus that drives an AC motor via a voltage signal of rectangular waveform. The drive control apparatus, which may be used in, for example, electric vehicles, controls the voltage signal applied to the AC motor such that the torque generated by the AC motor corresponds to a torque command value T* produced by an electronic control unit (ECU) (not shown).
A motor 2, which is in the form of a permanent magnet synchronization type AC motor, is connected to an inverter 4. The inverter 4 receives electric power from a battery (not shown), and supplies current to the stator windings of the U, V and W phases of the motor 2. A rectangular wave generating unit 6 is connected to the inverter 4. The rectangular wave generating unit 6 generates a switching (SW) signal for producing rectangular wave voltage with respect to each of the U, V and W phases. On the basis of the SW signals thus supplied, switching operations of the inverter 4 are controlled.
The rectangular wave generating unit 6 controls the phase of each SW signal, based on a voltage phase command xcex94xcfx86 determined by a PI computing unit 8 and a rotor angle xcex8 that is output from a resolver 10 provided adjacent to the motor 2.
For ease of discussion, a d-q coordinate system (magnetic pole coordinate system), rather than the quantities of the three phases U, V, W of a motor, is used to describe how a motor is controlled. The d-q coordinate system is fixed to a rotor of the motor in question, and voltage equations in a steady state of the motor are expressed in the d-q coordinate system as follows.
Vd=Rxc2x7Idxe2x88x92xcfx89xc2x7Lqxc2x7Iqxe2x80x83xe2x80x83(1)
Vq=Rxc2x7Iq+xcfx89xc2x7Ldxc2x7Id+xcfx89xc2x7"psgr"xe2x80x83xe2x80x83(2)
In the above equations, Vd and Vq represent a d-axis component and a q-axis component of voltage applied across the stator, and Id and Iq represent a d-axis component and a q-axis component of current passing through the stator, while Ld and Lq represent d-axis inductance and q-axis inductance. Also, xcfx89 represents the angular velocity of the rotor; and "psgr" represents the flux linkage. The direction of the current vector (Iq, Id) changes in accordance with the direction of the voltage vector (Vq, Vd). The value Iq, which contributes to the torque T of the rotor, also changes in accordance with the direction of the voltage vector.
The voltage phase command xcex94xcfx86 specifies the direction of the voltage vector, and is determined by the PI computing unit 8 so that the Iq provides a desired torque T. Hereinafter, a voltage phase command taken with reference to the d axis (that is, the angle of the voltage vector with respect to the d axis) is expressed as xcex94xcfx86, and a voltage phase command taken with reference to the q axis (that is, the angle of the voltage vector with respect to the q axis) is expressed as xcex94xcfx86xe2x80x2. The voltage phase commands xcex94xcfx86, xcex94xcfx86xe2x80x2 have the following relationship (3):
xcex94xcfx86=xcex94xcfx86xe2x80x2+90xc2x0xe2x80x83xe2x80x83(3)
When xcex94xcfx86xe2x80x2 is equal to 0xc2x0, the torque T is equal to 0. When xcex94xcfx86xe2x80x2 is equal to +90xc2x0, a maximum positive torque can be obtained. When xcex94xcfx86xe2x80x2 is equal to xe2x88x9290xc2x0, a maximum negative torque can be obtained.
In the d-q coordinate system, a desired torque T can be related to or associated with the voltage phase command xcex94xcfx86 (or xcex94xcfx86xe2x80x2). However, motor control is actually based on the quantities of the three phases U, V, W of the motor. Specifically, the phases of current supplied from the inverter 4 to the windings of the U, V and W phases of the motor 2 change depending on the rotor angle xcex8 and the voltage phase command xcex94xcfx86. More specifically, the current of each phase is a function of the sum ("xgr") of xcex94xcfx86 and xcex8xe2x80x2, where xcex8xe2x80x2 represents an electrical angle that is associated with a mechanical rotational angle xcex8 of the rotor. Since the quantities of the three phases change in accordance with the rotor angle xcex8, the rectangular wave generating unit 6 receives information regarding the rotor angle xcex8 from the resolver 10, and controls the phases of the SW signals corresponding to the three phases of the motor as described above.
Further, the torque T which is currently being produced by the motor is estimated based on the electric power supplied to the motor 2. An electric power computing unit 12 calculates the electric power supplied to the motor 2 based on the winding currents Iv, Iw of the V and W phases, which are obtained from respective current sensors 13, the rotor angle xcex8, which is obtained from the resolver 10, and the voltage phase command xcex94xcfx86. The U, V and W phases shift 120xc2x0 in phase from one another, therefore, the total sum of the currents Iu, Iv, Iw of the three phases is equal to zero in principle. Thus, the current sensors 13 are provided only for two phases (V and W phases in this embodiment), and the electric power computing unit 12 calculates the current value of the remaining phase (the U phase), based on the current values of the two phases measured by the sensors 13. The currents of the three phases can be expressed as follows, where I represents the amplitude of current.
Iu=Ixc2x7sin("xgr"+90xc2x0)xe2x80x83xe2x80x83(4)
Iv=Ixc2x7sin("xgr"+90xc2x0xe2x88x92120xc2x0)xe2x80x83xe2x80x83(5)
Iw=Ixc2x7sin("xgr"+90xc2x0+120xc2x0)xe2x80x83xe2x80x83(6)
The electric power computing unit 12 determines "xgr" from xcex94xcfx86 and xcex8. Here, the inverter 4 is supposed to generate rectangular waves that switch between a voltage level of xe2x88x92Vb/2 and a voltage level of Vb/2. The value Vb is transmitted from the inverter 4 to the electric power computing unit 12. The electric power computing unit 12 calculates voltage fundamental waves Vu, Vv, Vw contained in the rectangular wave voltages of the respective three phases, according to the following equations (7), (8) and (9).
Vu=(Vb/2)(4/xcfx80)xc2x7sin("xgr"+90xc2x0)xe2x80x83xe2x80x83(7)
Vv=(Vb/2)(4/xcfx80)xc2x7sin("xgr"+90xc2x0xe2x88x92120xc2x0)xe2x80x83xe2x80x83(8)
Vw=(Vb/2)(4/xcfx80)xc2x7sin("xgr"+90xc2x0+120xc2x0)xe2x80x83xe2x80x83(9)
Then, the electric power computing unit 12 calculates an estimated power P based on the following equation (10).
P=Vuxc2x7Iu+Vvxc2x7Iv+Vwxc2x7Iwxe2x80x83xe2x80x83(10)
A torque estimating unit 14 determines an estimated value of current torque T from the estimated power P determined by the electric power computing unit 12, and the speed N of rotation of the motor 2, according to the following equation (11).
T=keffxc2x7P/xcfx89xe2x80x83xe2x80x83(11)
In the above equation, keff is a correction factor of the efficiency of conversion for electric power to torque. The angular velocity xcfx89 is calculated from the rotation speed N of the motor 2. The rotation speed N is detected by a rotation speed detecting unit 16. The rotation speed detecting unit 16 detects the rotation speed N based on changes in the rotor angle xcex8 (output from the resolver 10) over time.
The estimated torque T is input to an adder 18. The adder 18 determines a torque deviation xcex94T according to the following equation (12) based on the torque command value T* and the estimated torque T. Then, the PI computing unit 8 performs torque feedback control so as to converge the torque deviation xcex94T to zero through PI control. Thus, the PI computing unit 8 determines the voltage phase command xcex94xcfx86xe2x80x2 (or xcex94xcfx86).
xcex94T=T*xe2x88x92Txe2x80x83xe2x80x83(12)
FIG. 6 is a diagram indicating an example of a voltage vector. An end point of a voltage vector 30 (having a length of Av) moves along a circle 32 in accordance with the value of the voltage phase command xcex94xcfx86xe2x80x2. Here, the voltage phase command xcex94xcfx86xe2x80x2 takes a positive value when it moves in the clockwise direction as viewed in FIG. 6.
Further, the winding wire resistance R of the motor 2 is normally small, and therefore the above-indicated equations (1) and (2) can be approximated to simpler ones by ignoring the terms that include R. Furthermore, the following equations (13) and (14) are obtained from the diagram of FIG. 6.
Vd=xe2x88x92Avxc2x7cos xcex94xcfx86xe2x80x2xe2x80x83xe2x80x83(13)
Vq=Avxc2x7sin xcex94xcfx86xe2x80x2xe2x80x83xe2x80x83(14)
Accordingly, the following equations can be obtained from the equations (1) and (2).
Id=(Avxc2x7sin xcex94xcfx86xe2x80x2xe2x88x92xcfx89xc2x7"psgr")/(xcfx89xc2x7Ld)xe2x80x83xe2x80x83(15)
Iq=Avxc2x7cos xcex94xcfx86xe2x80x2(xcfx89xc2x7Lq)xe2x80x83xe2x80x83(16)
The component Iq of the current vector contributes to the torque T. As the component Iq increases, the torque T produced increases. From the equation (16), it is understood that the torque monotonously increases as the voltage phase command xcex94xcfx86xe2x80x2 varies from xe2x88x9290xc2x0 to +90xc2x0, and that the maximum positive torque is produced when xcex94xcfx86xe2x80x2 is approximately equal to 90xc2x0, while the maximum negative torque is produced when xcex94xcfx86xe2x80x2 is approximately equal to xe2x88x9290xc2x0. If xcex94xcfx86xe2x80x2 exceeds either one of the limit values 90xc2x0, xe2x88x9290xc2x0, the absolute value of the torque produced decreases. Therefore, if the voltage phase command xcex94xcfx86xe2x80x2 is allowed to exceed xc2x190xc2x0, the torque feedback control may not be accomplished, namely, the torque deviation xcex94T may fail to be equal to zero. A limiter 20 is provided for preventing this possibility. That is, if the voltage phase command xcex94xcfx86xe2x80x2 output from the PI computing unit 8 exceeds xc2x190xc2x0, the limiter 20 clips the excessive value, so that control operations at later stages, including the rectangular wave generating unit 6 and others, will be performed with the voltage phase command xcex94xcfx86xe2x80x2 held in the range of xe2x88x9290xc2x0xe2x89xa6xcex94xcfx86xe2x80x2xe2x89xa690xc2x0.
As described above, the known drive control apparatus for driving an AC motor by using a voltage signal of rectangular waveform includes rotor angle detecting means, such as the resolver, and uses the detected rotor angle xcex8 for driving control. Namely, since the known apparatus structurally requires a rotor position sensor, the construction of the apparatus is likely to be complicated, resulting in an increased cost. If no rotor position sensor is provided, the known drive control apparatus becomes unable to perform control using a voltage signal of rectangular waveform.
It is one object of the invention to provide a drive control apparatus that permits control of a voltage signal of rectangular waveform to be applied to an AC motor, without requiring detection of the position of a rotor of the motor.
To accomplish the above and/or other object(s), there is provided according to one aspect of the invention a drive control apparatus for driving an AC motor by applying a rectangular wave voltage thereto, which includes: (a) a torque estimating unit that obtains an estimated torque of a rotor of the AC motor, (b) a torque deviation detecting unit that detects a torque deviation as a difference between the estimated torque and a torque command value representing a required torque of the AC motor, and (c) a rectangular wave voltage controller that controls a state of the rectangular wave voltage applied to the AC motor, through torque feedback control based on the torque deviation, so that a torque of the rotor approaches the torque command value. In the apparatus, the rectangular wave voltage controller detects a current rotation speed of the AC motor, and switches, in a predetermined order, a plurality of predetermined rectangular wave voltage states corresponding to different phase values of the rectangular wave voltage. Accordingly, the switching timing of the rectangular wave voltage states is set to a timing that deviates, from a reference timing determined based on the current rotation speed, by a length of time corresponding to the torque deviation.
When the torque deviation is equal to zero, switching of the rectangular wave voltage states is effected in the reference timing. When the torque deviation is not equal to zero, on the other hand, the switching timing is shifted from the reference timing by the length of time corresponding to the torque deviation. In this embodiment, the rectangular wave voltage states respectively correspond to mutually different phase values, namely, voltage vectors having mutually different directions. When the torque deviation is equal to zero, which means that the estimated torque coincides with the torque command value, the electrical angle formed between the rotor and the voltage vector of the rectangular wave voltage applied to the stator windings of the AC motor is equal to a constant value corresponding to the torque command value. In this case, an angular change in the electrical angle of the rotor is, in principle, equal to an angular change in the voltage vector of the rectangular wave voltage. In the meantime, an angular change in the electrical angle of the rotor is related or associated with a change in the mechanical angle of the rotor, and the time required for the change in the mechanical angle is determined depending upon the rotation speed of the AC motor. Further, an angular change of the voltage vector between one of the plural rectangular wave voltage states and the next rectangular wave voltage state is known. Accordingly, when the torque deviation is equal to zero, the time between the beginning of the certain rectangular wave voltage state and that of the next rectangular voltage state is determined depending upon the rotation speed of the AC motor. Therefore, the reference timing, discussed above, is determined based upon the current rotation speed of the AC motor. If the switching timing of the rectangular wave voltage states is shifted from the reference timing, the electric angle formed between the rotor and the voltage vector of the rectangular wave voltage applied to the stator windings of the AC motor changes, resulting in a change in the torque generated at the rotor. By controlling an amount by which the switching timing is shifted from the reference timing according to the torque deviation, the torque feedback control can be accomplished without knowing or acquiring the rotating position of the rotor.
In a preferred embodiment of the invention, the switching timing controller determines an amount of rotation of the rotor during the current rectangular wave voltage state, according to the torque deviation, and calculates a time duration between switching to the current rectangular wave voltage state and switching to the next rectangular wave voltage state, based on a ratio of the amount of rotation of the rotor to the rotation speed.
As discussed above, when the torque deviation is equal to zero, the amount of rotation of the rotor during the current rectangular wave voltage state is determined depending upon a difference between the phase values of the current rectangular wave voltage state and the next rectangular wave voltage state. When the torque deviation is not equal to zero, the switching timing controller increases or reduces the amount of rotation of the rotor obtained in the case where the torque deviation is equal to zero, depending upon the torque deviation. By dividing an angle of rotation thus obtained by the current angular velocity of the AC motor, the controller determines a time duration between the beginning of the current rectangular wave voltage state and the beginning of the next rectangular wave voltage state.
According to another aspect of the invention, there is provided a drive control apparatus for driving an AC motor by applying a rectangular wave voltage thereto, which includes (a) an electric power estimating unit that obtains an estimated electric power of the AC motor based on a command value of the rectangular wave voltage applied to the AC motor, (b) a torque estimating unit that obtains an estimated torque of a rotor of the AC motor, based on the estimated electric power, (c) a torque deviation detecting unit that detects a torque deviation as a difference between the estimated torque and a torque command value representing a required torque of the AC motor, and (d) a rectangular wave voltage controller that controls a state of the rectangular wave voltage applied to the AC motor, through torque feedback control based on the torque deviation, so that a torque of the rotor approaches the torque command value.
As shown in equation (11) above, the value of the current torque which is needed for torque feedback control can be estimated from the electric power consumed by the AC motor. According to the above aspect of the invention, the estimated electric power of the AC motor is obtained using a command voltage value of rectangular wave voltage, instead of voltage fundamental waves calculated based on the position of rotation of the rotor. Thus, the electric power can be estimated without knowing the rotational position of the rotor, and the torque can be estimated based on the estimated electric power.
According to a further aspect of the invention, there is provided a drive control apparatus for driving an AC motor by applying a rectangular wave voltage thereto, which includes: (a) a rectangular wave voltage controller that switches, in a predetermined order, a plurality of predetermined rectangular wave voltage states corresponding to different phase values of the rectangular wave voltage, and (b) a rotating speed estimating unit that estimates a rotation speed of the AC motor, based on a time duration between switching to one of the rectangular wave voltage states and switching to the next rectangular wave voltage state, and a difference in the phase value between the switching rectangular wave voltage states.
In the drive control apparatus capable of performing torque feedback control as described above, when the torque deviation is equal to zero, a difference in the phase value between the rectangular wave voltage states, namely, an angular change in the voltage vector, is equal to an angular change in the electrical angle of the rotor, which can be converted into a mechanical angular change of the rotor. In this case, the mechanical rotational angle of the rotor within the time duration or interval of switching of the rectangular wave voltage states is obtained from a difference in the phase value between the rectangular wave voltage states, and the rotating speed of the AC motor is determined by dividing the mechanical rotational angle of the rotor by the switching interval of the rectangular wave voltage states. When the torque deviation is not equal to zero, the switching timing of the rectangular wave voltage states is shifted, therefore, an angular change in the voltage vector between the rectangular wave voltage states is not equal to an angular change in the electrical angle of the rotor. In this case, the rotational speed may be obtained in view of the degree by which the switching timing is shifted, or an approximate rotational speed may be obtained without considering the shifted timing. Thus, the rotating speed of the AC motor used for torque feedback control or calculation of the estimated torque can be estimated without detecting the rotational position of the rotor.
According to a still another aspect of the invention, there is provided a drive control apparatus for driving an AC motor by applying a rectangular wave voltage thereto, which comprises: (a) a torque estimating unit that obtains an estimated torque of a rotor of the AC motor, (b) a torque deviation detecting unit that detects a torque deviation as a difference between the estimated torque and a torque command value representing a required torque of the AC motor, (c) a rectangular wave voltage controller that controls a state of the rectangular wave voltage applied to the AC motor, through torque feedback control based on the torque deviation, so that a torque of the rotor approaches the torque command value, and (d) a torque command value limiter that limits the torque command value within a range that permits the torque feedback control to bring the torque of the rotor close to the torque command value.
The torque generated by the rotor changes depending upon an angle formed between the electrical angle position of the rotor and the voltage vector of the rectangular wave voltage state applied to the stator windings. When the angle is approximately equal to 90xc2x0, the maximum torque is produced. On the other hand, if the angle exceeds the upper limit of 90xc2x0, the torque is reduced, and the torque deviation is not converged to zero through torque feedback control. According to the above aspect of the invention, the torque command value is limited or restricted so as not to produce a torque deviation corresponding to an angular range that exceeds the upper limit.